To reduce fuel consumption and exhaust emissions, the use of hybrid drives in motor vehicles is increasing. A hybrid drive unit comprises a first drive-power source in the form of an internal combustion engine or combustion motor, and a second drive-power source preferably in the form of an electric machine. The second drive-power source co-operates with an energy accumulator, which is more predominantly charged when the second drive-power source is operating as a generator and more predominantly discharged when the second drive-power source is operating as a motor. The energy accumulator can be for example an electric energy accumulator, a mechanical energy accumulator such as a flywheel mass accumulator, or even a hydraulic energy accumulator.